The present invention relates generally to noise suppression systems, and more specifically the invention pertains to a delayed differential noise cancellation system.
It has been recognized for some time that excess noise (i.e. noise above the shot noise level), spurious modulation and power drift in the laser can significantly reduce the accuracy of many measurements of laser light. In gas lasers, the noise levels can easily reach 50 dB above shot noise even at relatively high frequencies.
Shot noise is a random current fluctuation that occurs when light is detected. Conventional photodetectors, such as PIN diodes, pass current from their anode to their cathode in proportion to the number of photons that strike the detector. Shot noise results from the random arrival of photons, which can be modeled as a Poisson process. This random arrival time causes an electric current with an average value and a standard deviation in any given time interval. Excess noise represents current fluctuations larger than expected from this standard deviation, resulting from photons that do not arrive randomly. A method of reducing noise is to take a sample of the output beam and apply negative feedback to the laser operating current or to an external optical attenuator to keep the photocurrent, derived from the sample beam, constant. These systems tend to be complicated or expensive and can, at best, bring the signal-to-noise ratio of the output beam up to the signal-to-shot-noise ratio of the sample beam. Since, in these systems, the sample beam is usually appreciably weaker that the output beam and, so contains relatively more shot-noise, this method may not provide acceptable levels of noise reduction. In addition, since these systems depend on feedback, the effective bandwidth of the noise-reduced beam is often relatively small.
The following patents, which are incorporated herein by reference, describe prior art noise suppression techniques:
U.S. Pat. No. 5,428,314, Jun. 27, 1995, Odd/even order distortion generator and distortion cancellation circuit; Van Cleave, Todd;
U.S. Pat. No. 5,239,401, Aug. 24, 1993, Optical modulator for cancellation of second-order intermodulation products in lightwave systems;
U.S. Pat. No. 5,227,857, Jul. 13, 1993, System for cancelling phase noise in an interferometric fiber optic sensor arrangement; and
U.S. Pat. No. 5,134,276, Jul. 28, 1992, Noise cancelling circuitry for optical systems with signal dividing and combining means.
The present invention is a narrow band optical noise cancellation system. One embodiment of this system includes:
a continuous wave optical laser that generates an optical carrier signal which has a modulating period; an RF source of a radio frequency modulating signal; a modulator that outputs a first and second optical modulated signal by splitting the optical carrier signal into a first and second carrier signal and modulating them with the radio frequency modulating signal; a means for delaying the first optical modulated signal by an odd multiple of 180xc2x0 degrees of the modulating period to output a delayed first optical modulated signal at a first power level; an optical attenuator that adjusts the second optical modulated signal so that it has a second power level that equals the first power level of the delayed first optical modulated signal from the delaying means to output thereby an output modulated optical signal; and a photodetector that acts as a means for converting the output modulated optical signal into a modulated electrical signal.
In the embodiment of the invention, the modulator includes: an optical splitter that splits the optical carrier signal into the first and second carrier signal, a phase shifter that modulates the relative phase of the first and second carrier signal with the radio frequency modulating signal, and an optical combiner that sums the first and second carrier signal and produces a first and second processed optical carrier signal.
Another embodiment of the invention is a narrowband optical noise cancellation system that includes:
a diode laser that acts as a means for generating an optical carrier signal which has a modulating period;
a first and second RF source of first and second radio frequency modulating signals;
a linearized modulator unit that modulates the optical carrier signal with the first and second radio frequency modulating signals to output thereby a first and second optical modulated signal;
a means for delaying the first optical modulated signal by an odd multiple of 180xc2x0 degrees of the modulating period to produce a delayed first optical modulated signal; and
a means for combining the delayed first optical modulated signal with the second optical modulated signal to output a combined optical modulated output signal.
In the embodiment, the linearized modulation unit includes:
an optical splitter that splits the optical carrier signal into a first and second optical carrier signal;
a first modulator which modulates the first and second optical carrier signal with the first radio frequency modulating signals to output a first and second processed optical carrier signal;
a second modulator which modulates the first and second processed optical carrier signal from the first modulating laser with the second radio frequency modulating signal to output thereby the first and second optical modulated signal.
It is an object of the invention to provide a noise cancellation system for use in an optical modulation system.
It is another object to provide a noise cancellation process for use with optical modulation systems.